**3. Results: Feature-Based Framework for Inspection**

As already mentioned, product specification involves:


characteristic requires the definition of a GPS operator that establishes the procedure to obtain it from the data of the involved geometrical elements.

c. The validation whether the total assembly performance (tolerance) meets the functional condition. This is calculated through a chain that considers the characteristics of the assembly components (individual parts) and the contact conditions.

Similarly, for each specification to be verified, the inspection plan specification involves:


As is clear, in product specification and inspection plan specification very similar tasks must be undertaken. Furthermore, both specification exercises work on a common representation of the geometry with defects of the part, either of the conceived or of the real one. GPS operators are applied to the geometry with defects in order to quantify the characteristics and their variability. When the operators used for both exercises are coincident (duality), then uncertainty is minimised.

Hence, the specification of the inspection process plan involves establishing GPS operators on the verification geometries of both the part and the components of the inspection resource and analyzing the contacts between the previous geometries. Therefore, in the first part of this section the specific geometries needed for verification (part and inspection resource) are going to be studied. In the next two parts of this section, an inspection feature and assembly models are proposed. These models are based on the verification geometries. Finally, a case study is presented.

#### *3.1. Geometry Model for Verification*

Usually, in product specification the designer considers skin and/or skeleton models with defects for a GD&T analysis process based on simulation [29]. These models are constructed from the nominal model based on ideal geometries with imagined dimensional and angular defects. As shown in Figure 6, these imagined geometries with defects are represented by the Substitute and Extracted Features defined in relation to a reference geometry represented by the Reference Feature, which usually is the same as the Nominal Feature.

However, in the specification of the inspection plan the planner considers skin and/or skeleton models with defects imagined as a result of the extraction process. The type of these imagined extracted geometries depends on the type of extracted geometry that the inspection resource can provide.

#### **The Geometry of the Part**

In order to verify a part characteristic, the inspection planner must have an adequate representation of the part real geometry. This representation should enable the planner to establish the verification GPS operator, as a set of several GPS operations (partition, extraction, association, etc.). This GPS operator will include a last evaluation operation to allow for the verification of the

characteristic. The representation of the part geometry, gathered during the measurement process, is referred in this work as "verification primitive model". Hence, the GPS operator established by the planner will operate on this verification primitive model transforming it into simpler ones from which the required linear and angular distances to complete the evaluation operation of the characteristic can be computed.

The verification primitive model for inspection plan specification, unlike product specification, is very often a discrete model, obtained by sampling a finite number of points, segments or tessellation elements on real part surfaces. The vast majority of specialised literature, including the GPS standard, assume that inspection plan specification starts with this type of verification primitive model (discrete model).

However, this situation in only present when metrological systems based on coordinate measuring processes equipped with (mechanical and optical) probes are used. When basic metrology, such as a calliper, is employed, the verification primitive model is a much simpler one, since from the available information only an ideal profile model with dimensional defects (due to linear and/or angular variations) can be obtained. As Figure 6 shows, this profile model does not consider part form and orientation defects, since these are neglected by the contact surfaces of the inspection resource assumed geometrically perfect.

**Figure 6.** Part geometry models for verification.

Then, the verification primitive model is a representation of a real instance of the part geometry that depends on the extraction method and the inspection resource used, and can be of two main different types (Figure 6, left):

1. Discrete models with defects (integral or derived profiles/surfaces) consisting of sets of points, segments or tessellation elements (with a particular pattern). These models are obtained when measurement is performed by equipment that provides coordinate information, such as CMM (Coordinate Measurement Machine), optical equipment, surface form/texture metrology, etc. The coordinate information is referred to the equipment coordinate system that is realised by the movements of its guideways. To make this equipment very flexible, its guideways can be linear, resulting in rectangular coordinate systems, or a combination of linear and angular movements, resulting in spherical, cylindrical, etc., coordinate systems. There is equipment with two guideways that can be used to obtain two dimensional discrete models and others with three or more guideways that can be used when three dimensional discrete models are required.

2. Ideal models with dimensional defects that keep the nominal form. These models are obtained when measurement is performed either by conventional equipment (calliper, micrometer, goniometer, etc.) or by equipment and set-ups used in comparison measurements. The first ones provide a specific linear or angular distance between two ideal geometries that are embodied by the measurement equipment. The second ones provide two linear or angular distances (maximum and minimum deviations) that enable the construction of two ideal geometries (surfaces or profiles) that are internally or externally enveloping the real part geometry. The construction of these two ideal enveloping geometries is performed by the movement of the measurement equipment guideways (sweeping movement). When surface models (3D) are required, it will be necessary to use two axes for the sweeping movement (two isoparametric lines), resulting in an enveloping surface. However, if plane profile models (2D) are desired, just one axis for the sweeping movement will be required, resulting in an enveloping line. When surface models of a complete partitioned geometry using any of these two types of measurement processes (conventional equipment or set-ups for comparison measurements) are desired, measurements in several planes (parallel, coaxial, etc.) will be required in order to cover the whole partitioned surface. Obviously, the uncertainty of these surface models with dimensional defects will depend on the possibility of coincidence of the reference geometry with these profiles, as it will be explained in the next section.

From these primitive models, the verification GPS operator can establish other simplified models (Figure 6, right) required to assess the part specified characteristics. In particular, the verification operator can establish the following simplified continuous surface/profile geometric models with defects: (a) Non-ideal models, which are generated by reconstruction operations (fitting and interpolation) from the primitive models with the aim of obtaining the points that match with the sampling points established in the specification; (b) Ideal models with angular defects; (c) Ideal models with linear dimensional defects; and (d) Ideal derived models that can be obtained either by a GPS derivation operation from the previous simplified models or directly from a derived primitive model resulting from a measurement process.

Figure 6 also shows that when the primitive models are ideal, they are the same as the corresponding simplified models (b–d). Although it is often unnoticed, very often the primitive geometry itself already contains information (measurements) about the specified characteristics and, therefore, no subsequent transformation of the geometry will be necessary to obtain these characteristics. This is the case of many dimensional characteristics associated with a specific geometrical element that are obtained by direct measurements of dimensions (angle, diameter, width, etc.) or by sweeping processes. When using sweeping processes, the measurement process or equipment does not register deviations of specific points of the geometrical element of the part, and provides only the total deviation produced in the sweeping process of specific profiles or of the complete surface. A classic example of this type is the measurement of the straightness of a plane using a rule and an indicator.

#### **The Geometry of the Inspection Resource**

In addition to the real part geometry, in verification, an inspection resource is also involved in the measurement process. This inspection resource has real geometries of high quality that are always assumed to be ideal, neglecting their defects, since they are usually very small. Examples of these geometries are the surface of a plate, the axis of a chuck, etc. Based on this assumption, the real geometries of the inspection resource are represented using ideal models (without defects) for all the reasoning and computing processes required to obtain the measured characteristic. The uncertainty of the measured characteristic is influenced by the quality of the real geometries assumed as perfect.

Generally, in order to obtain a measured characteristic between two geometries (target and datum) the comparison of the real target geometry in relation to the datum frame geometry (specification reference geometry) is required. This comparison involves obtaining linear and angular distances in

relation to this specification reference geometry. In turn, this specification reference geometry can also be considered as a target geometry, whose measurement involves comparing it with another datum frame geometry (measurement reference geometry).

Therefore, every measurement reference geometry can be used as a specification reference geometry. The measurement reference geometry, in relation to which linear and angular distances are obtained, is always realised by the inspection resource. This realization, as it will be explained later in this section, can be of different types, such as a flat surface contact, an axis of a revolved surface, etc. On the other hand, the specification reference geometry, which is always required for the measurement of a specified characteristic, is obtained either by a measurement process comparing it with a measurement reference geometry, or by doing it coincident with a measurement reference geometry embodied by the inspection resource using an alignment process.

In general, these three geometries (tolerance geometry, specification reference geometry and measurement reference geometry) are involved in the measurement of a specified characteristic (Figure 7). According to the specified characteristics and the selected measurement process, some of these geometries are the same. For example, when form characteristics are verified, the specification reference and the target reference can be the same. When orientation and location characteristics are verified, the specification reference and the measurement reference can be the same.

**Figure 7.** Geometries involved in verification.

The real geometries of the inspection resource considered as ideal models are normally known as embodiments in the metrological domain. The linear or angular distance values obtained by the inspection resource are always referred to these embodiments that are the reference for the measurements. Embodiments to establish the measurement reference can be also other ideal geometries that are of the same type to the previous ones (real geometries of the inspection resource). They are usually an offset of the real ones and are established during the equipment set-up process. For example, when a parallelism specification is inspected by means of a set-up using a surface plate, a height gage and a dial indicator, the reference measurement can be the surface plate itself contacting the specification reference. However, an imaginary plane parallel to the surface plate with a specific offset controlled by the height gage could also be used as the measurement reference.

The embodiment of the "measurement references" by the inspection resource can be of one of the following types:


measurement equipment guideways has to provide the minimum number of independent axes required by the type of tolerance geometry.

c. Calculated embodiment, when the reference is obtained by mathematical association operations using the part extracted points, segments or tessellation elements and appropriate criteria such as least square, minimum outer diameter, etc.

Not all types of references embodied by the inspection resource can be used with all types of primitive models of the tolerance geometry. In particular, the reference as calculated embodiment (c) leads to discrete part primitive models of the tolerance geometry, which can be simplified to ideal geometries if appropriate. On the other hand, kinematic embodiments (b) or positioning embodiments (a) lead to an ideal part primitive model of the tolerance geometry. More specifically, the ideal part primitive model obtained using a kinematic embodiment is a set of two ideal geometries enveloping the real geometry. These two ideal enveloping geometries are of the same type and are generated simultaneously with the kinematic embodiment geometry. However, the positioning embodiment leads to an ideal part primitive model that is an ideal geometry establishing a single boundary (external or internal) of the real geometry.

As has been mentioned, the measurement reference is embodied by the inspection resource, whereas the tolerance geometry to be extracted exists on the part. In addition, to obtain the measured characteristic a specification reference also existing on the part is required. This specification reference must be located (usually by coincidence) in relation to the reference embodied by the inspection resource. This is the so-called alignment process that always introduces an additional uncertainty in the inspection process. If a misalignment between the real geometry of the part and the reference geometry appears, a misalignment error is also present.

The aim of the alignment process is basically to make two geometries, one of the parts and one of the inspection resources, coincident (orientation and situation). The measurement reference geometry is realised by real geometries of the resource (high precision surface plates, gusset plates, mandrels, etc.) known as simulated datum. The defects of these real geometries of the resource are neglected compared to the part geometry defects and, therefore, they are considered to be ideal geometries. It must be noticed that the effect of this assumption is included in the resource uncertainty obtained during the calibration process. The lower the quality of the real geometries of the inspection resource, the higher the measurement (implementation) uncertainty. The part geometry must contact with these real (assumed ideal) geometries of the inspection resource. However, since part geometry is not ideal, there is no one single stable solution for the contact. Due to the significant effect of this circumstance on the uncertainty, the use of some requirements to rule the relative location is required, such as the minimum requirement or the minimum rock requirement [30].

Very often, the alignment process is realised locating the part by physical contacts with the inspection resource minimising the deviations between part and inspection resource geometries. In these alignments by physical contact, two cases can be distinguished depending on whether the part contact surface is the same or not as the specification reference. An example of the first case is when a part flat surface directly contacts with the surface plate that orients the part and is used as reference. An example of the second case is the clamping process of a cylindrical part using a roundness measuring instrument where dial indicator values on the cylindrical surface when turning the part around the equipment axis are minimised.

In coordinate-based measurement processes, the alignment process is the calculation of the measurement reference. In this case, the alignment process involves calculating an ideal geometry that is used as specification/measurement reference and probing on its normal direction.

#### *3.2. Inspection Feature Model*

In this section, and based on the concepts related to the geometries with defects explained in the previous section, an Inspection Feature Model is proposed and described using UML diagrams. The Inspection Feature is defined as a subtype of the Application Feature considered in the UAF framework outlined in section.

The *Inspection Feature* (InspF) shown in Figure 8, as subclass of the *Specification Feature* class, is an aggregation of the classes with the information about the structure (*Inspection Structure*), the geometric interface (*Inspection Geometry Feature*) and the functional geometric condition (*Inspection Condition*), which are established as requirements on the characteristics to be measured (*Characteristic Measurement*) in order to obtain the values of the specified characteristics (*Inspection Requirement*) that points to the self-geometries of the InspF. These specified characteristics have been established along the product design stating their GPS specification operators and their variation limits (tolerances).

**Figure 8.** Inspection Feature (InspF) Model.

The inspection process plan specification starts analyzing those specified characteristics in order to define the part geometry using the feature types from the InspF Library (feature recognition) and to establish the *Inspection Condition*. The part recognition using the InspF Library developed as stated in the methodology section, is essential to ensure: (1) that is possible to extract the measurement data for the specified characteristic calculation and (2) that there exists inspection resource type able to execute the data extraction. These inspection resource types facilitate the selection of one or more technical solutions to carry out the InspF measurement.

In the same way that *Inspection Condition* relates the InspF with the product functional structure, the *Inspection Structure* relates the InspF, and more specifically its *Nominal Feature*, with the component structure of the planned inspection assembly, in which part participates. For this, the *Inspection Structure* contains the topological structure of the InspF and positions it in the part framework.

The *Inspection Geometry Feature* aggregates two feature: (1) The *Nominal Feature*, which represents the nominal geometries of the feature that are defined as ideal geometries; and (2) The *Measurement* *Defects Feature*, which is used to represent the real geometries participating in the measurement process as ideal geometries that model form and location (orientation and situation) defects. The *Measurement Defects Feature* aggregates three features: (1) The primitive geometries extracted in the measurement process (*Measurement Extracted Feature*); (2) The reference geometries (datum frame) used to obtain the previous ones (*Measurement Reference Feature*); and (3) The required geometries resulting from simplification processes applied on the primitive geometries (*Measurement Substitute Feature*).

The extracted geometry with defects (*Measurement Extracted Feature*), which is a representation of the real geometry obtained as described in Section 3.1, can correspond to discrete primitive models (*Discrete Extracted Geometry Feature*) or to ideal primitive models (*Enveloping Extracted Geometry Feature*) as an envelope model, consisting of one or two ideal geometries limiting the real one.

As it has been previously explained in Section 3.1, the *Measurement Reference Feature*, which is the reference for the measured values, can be a *Positioning*, a *Kinematic* or a *Calculated Embodiment*. The *Measurement Reference Feature* can be any of the invariance classes geometries [27,29,31,32].

As Figure 8 shows, the *Inspection Condition* aggregates the characteristics to be measured (*Characteristic Measurement*). The *Characteristic Measurement* is an associative class that, in general, characterises the relation between *Measurement Defects Features*. This characterization is expressed according to Geospelling language as a set of sequenced GPS operations to establish and obtain the value of a characteristic (linear or angular distance) between any of the three components of the *Measurement Defects Feature* (Extracted, Substitute and/or Reference). The *Characteristic Measurements* can be of two main types: (a) *Extracted Characteristic Measurement*, which are characteristics between a *Measurement Extracted Feature* and a *Measurement Reference Feature* directly obtained by the inspection resource as linear or angular distances; and (b) *Calculated Characteristic Measurement*, which are characteristics between a *Measurement Substitute Feature* and either another *Measurement Substitute Feature* or a *Measurement Extracted Feature* obtained as linear or angular distances after applying mathematical/geometrical operations to values given by the inspection resource. The second type (b) of characteristic measurements are the most common ones when using inspection resources that provide a big amount of part geometrical data, such as the widely used coordinate-based measurement equipment. The latest standard developments in this field mainly focus on this type of measurement equipment.

Two types of *Extracted Characteristic Measurements* can be distinguished:


Similarly, two types of *Specified Characteristic Measurements* can be distinguished:


As has just been described, a key entity of the *Inspection Feature* is the *Measurement Defects Feature* that represents the real geometry of the part with defects through a combination of three geometries: the *Measurement Extracted Feature* and *Measurement Substituted Feature*, representing the defects on the part, and the *Measurement Reference Feature*, required in every measurement process for verification in order to orient and/or locate the first two. In addition, the model includes several associative classes to characterise, through GPS operators, the relationships between these three geometries, either for simplification and alignment purposes (*Substitution Operation* and *Alignment Operation*) or for the evaluation of the characteristic to be verified (all subtypes of *Characteristic Measurement*). The latter is related to the *Inspection Condition*, which is also included in the *Inspection Feature*.

#### *3.3. Inspection Assembly*

As mentioned in Section 2, the inspection planner task for the verification of a specific characteristic consists of defining an assembly, made up of the part and the inspection resource. This assembly must be able to extract the part geometric information required for the evaluation of the characteristic by a GPS operator. In addition, the planner must also validate that total uncertainty of the selected assembly is adequate for the limit established for the inspection condition. The extraction of the part geometric information, as it has been explained in the former section, involves the selection of reference surfaces in relation to which deviations, as linear or angular distances, are measured. On the other hand, the use of dual verification and specification operators will reduce the uncertainty.

The complete inspection process plan specification will include all the assemblies required to measure the InspF involved in the verification of all specified characteristics of the part. Obviously, in order to optimise the inspection process, the number of assemblies used should be minimised. Each assembly will require a set-up including the orientation and location of the part in the inspection resource, which has been previously referred as the alignment process. This alignment process can be more or less time consuming depending on the type of inspection resource and alignment and will have an influence on the uncertainty.

As previously mentioned, the established inspection assemblies are made up of two components (the part and the inspection resource) and two interactions exist between them. Each interaction includes all associative classes that describe the relations between part and resource features. The two interaction types are: (a) the location interaction, which holds, orients and/or positions the part in relation to the equipment reference system; and (b) the measurement interaction, which generates the stimulus, by contact or without contact between the probe and the part, for the registration of the sensors signals. Although the inspection resource or equipment itself is a mechanical assembly made up of several components and their interfaces, it will be considered as a whole (black box), characterized by a global uncertainty accompanying all the values of measurements carried out using that resource.

Figure 9 shows the model for the inspection assembly that enables the planner to analyze and specify the inspection process by reasoning on the assembly chains or loops (*Inspection Chain*). An *Inspection Chain* aggregates *Inspection Contacts*, which represent all the fixed location interactions between the part and the inspection resource defining the assembly architecture, and *Inspection Conditions*, that aggregate one or more *Characteristic Measurement*. Each *Inspection Chain* is useful to analyze one of the *Inspection Condition* that corresponds to an *Inspection Requirement*. Usually an *Inspection Condition* is related to measurement operations that results in measurement data. This type of *Inspection Condition* is a *Measurement Condition*. However, when the part is inspected using gages, only the conformance is checked, but no measurement data is available. This type of *Inspection Condition* is a *Gage Condition*.

**Figure 9.** Inspection Assembly Chain Model.

These chains allow for the planner the establishment and validation of the final solution through the analysis of the required D.o.f. (*Dof Chain*) and the uncertainties (*Uncertainty Chain*) introduced by the different involved elements. The uncertainty chain includes the uncertainties (*Inspection Uncertainty*) of all the relationships between geometrical measurement defects features of the part and of the inspection resource as a whole that have to be stacked up to fulfil the inspection condition. The D.o.f. chain includes the information about the required active and inactive D.o.f. (*Inspection Dof*) for part location, sweeping and measurement.

#### *3.4. Case-Study*

In this subsection, a simple case study is described with the aim of showing how the proposed Inspection Feature Model supports the reasoning carried out in some of the tasks typical of the inspection process planning. The example considers a very simple part (see central part of Figure 10) with just one key characteristic. This characteristic has been established using a standard position tolerance specification that restricts the deviation of the hole axis in relation to a datum defined by plane A and plane B.

**Figure 10.** Graph including the InspF for the case-study.

The specification of the process plan begins with the recognition of the toleranced geometry (cylinder and planes A and B) based on the InspF types established in the Library. In this case, the planner identifies the hole surface as one Cylinder InspF type and the two plane surfaces as two Plane InspF type. Additionally, taking the tolerance of the specified characteristic (0.2 mm) as basis, the planner establishes the Inspection Requirement with the statement: "To measure the deviation of the hole position with a maximum uncertainty of 0.03 mm". This uncertainty value complies with the 1/6 relation usually established between the specified tolerance and the uncertainty of the measurement process.

Once the functional requirement (Inspection Requirement) has been established, the planner must find a solution to measure the characteristic. Previously, however, he/she will have to define the Measurement Substitute Features (MSF) that are capable of obtaining and evaluating the measurement of position characteristic by the application of the required construction, calculation and evaluation GPS operations. In this case, the MSF defined are two, one corresponding to the Cylinder InspF and another one aggregating the two Planes InspF of the compound datum AB. When these MSF have been determined and taking into consideration the requirements compelled by the InspF types they belong to, different inspection solutions can be examined.

For this, several alternative *Measurement Reference Features* (MRF) for each MSF can be considered. Next, for each of these alternatives, a series of requirements must be established on the *Characteristic Measurements* necessary to fulfil the Inspection Requirement. These *Characteristic Measurements*, which constitute the *Inspection Condition*, are established in terms of uncertainties and D.o.f.

In particular, for this case study, the MDF that could be linked to the MSF corresponding to the hole and to the datum AB could be any of the types considered in the model (*Kinematic Embodiment*, *Calculated*, *Positioning Embodiment*). However, some of these MDF would be difficult to realise and should be disregarded. Furthermore, if as usual the MDF are kinematic embodiment or calculated, the *Measurement Extracted Features* (MEF) required to obtain the MSF should also be defined.

Usually, since several alternative MRF will have been defined for each InspF, the planner should study whether an MRF corresponds to more than one InspF, because the existence of MRF common to several InspF helps to minimize the number of required inspection assemblies (set-ups). In the case at hand, given its simplicity, it is clear that the two MSF can be obtained using a single inspection assembly and the following alternative solutions could be considered:


Although the specification of any of the three alternatives could be object of study, only the third is going to be analyzed. The analysis will be supported by the construction of the graph shown in Figure 10. Following the previously described procedure, MSF are first placed in the graph and later the MRF and the coordinate system of the resource are also placed. In this case, as the selection of solution has already been made, only one MRF is represented for each MSF, all of them of *Calculated* type. Thus, three MEF are also incorporated in the graph. These MEF correspond to the cylinder and the two planes. Proceeding with the graph construction, the thus far represented entities are linked by lines that symbolise the relationships established among the entities. In this case, two types of relationships can be established, i.e., *Distance Measurement* and *Substitution Operation*. The whole set of links is a graphical representation of the *Inspection Chain* that supports the identification of the involved uncertainties and D.o.f. chains.

As the graph shows, there are some entities that belong to the part (placed above the interface line) and others that belong to the resource (placed underneath the interface line). It can also be noted that there are some links that cross the interface line. These links are instances of the *Extracted Characteristic Measurement* and represent the measurement interaction between the part and the resource. In this case, there are no links representing the location interaction, since part location is not involved in the measurement.

Finally, although the objective of the present research work is not the development of an object-oriented application for process planning, as already mentioned in the methodology subsection, Figure 11 shows some instances of the entities and relationships defined in the case study for the Cylinder InspF using a UML object diagram. The aim is to help the reader in the comprehension of the case by detailing some of the attributes of the classes of the InspF Model. As can be seen in Figure 11, among the object attributes those required to build and analyze the D.o.f. and uncertainty chains can be found.

**Figure 11.** Some object instances and relations of the case study.

#### **4. Conclusions and Future Work**

In this work, a feature-based framework for inspection has been proposed. This framework is a specialization of a more general feature-based framework that supports the specification, analysis and validation of any technical solution (artefact). In this general framework, the Application Feature plays a key role since it is an informational object that carries the mapped functional and the structural solutions.

The development of the proposed feature-based framework for inspection has enabled to prove that the general feature-based framework is adequate not only for the specification, analysis and validation of GD&T characteristics on components of product artefacts (assemblies), but also for process artefacts (assemblies), more particularly for inspection assemblies. These inspection assemblies participate in the execution of the operations included in a set-up of the inspection plan. An inspection assembly (set-up) is made of two components: the subject part of inspection and all measurement devices (chucks, rules, plates, gages, probes, guideways, etc.) that together constitute the measurement resource.

As part of the feature-based framework for inspection, the Inspection Feature (InspF) is an essential element because it contains the necessary information to check the compatibility between the part and resource features allowing, as exposed in the included case-study, the specification and validation of inspection assemblies.

The results of this research show the possibilities of the proposed Inspection Feature for the development of knowledge-based applications in the field of inspection planning. The proposed model supports the design/selection of inspection solutions in collaborative production contexts, described in the introduction. However, from a conceptual point of view, additional work to validate the proposed approach is still needed. To that end, it is proposed, on one hand, to study in depth the inspection interaction from the resource perspective, and, on the other hand, to test the model consistency by stating an ontological model implemented in OWL (Ontology Web Language) and SRWL (Semantic Web Rule Language). In addition, the ontological approach will allow the incorporation of knowledge required to support process planning tasks, enabling the automated reasoning, the capture of new knowledge through the addition of new rules, etc.

**Author Contributions:** Conceptualization, F.R.S., P.R.C. and G.M.B.B.; Formal Analysis, F.R.S., P.R.C. and G.M.B.B.; Literature review, F.R.S. and S.B.N.; Supervision, F.R.S.; Validation, F.R.S., P.R.C. and G.M.B.B.; Writing-Original Draft Preparation, P.R.C. and G.M.B.B.; Writing-Review & Editing, G.M.B.B. and S.B.N.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

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